Flick urges Barca to return to 'unstoppable' best vs. BVB

Manager Hansi Flick has urged La Liga leaders Barcelona to "get back to what makes us unstoppable"  against  Borussia Dortmund, goalkeeper Inaki Pena told reporters on Tuesday.

The team's meet in the Champions League in Dortmund on Wednesday with both clubs among the pacesetters in the new look first round.

Flick's Barcelona were nine points clear of Real Madrid in La Liga at the start of November but a series of stumbles have allowed the Los Blancos, who have a game in hand, to cut the lead to two points.

Flick, who won the Champions League and several other titles during a short but successful stint in charge of Bayern Munich, adopts a high-risk approach. 

Pena said the approach had been difficult to adjust to when Flick joined Barcelona in the summer.

"We were a little shocked by the exaggerated idea that he had. It was a process that we had to integrate in pre-season," Pena said, but added "it's something that little by little you become convinced is better for you".

Flick told reporters the setbacks were normal for a young side. 

"We are a young team, it is clear that we cannot be at 100 percent in every game.

"It's normal to have a few setbacks in the season, but I'm completely confident in the team."

With three games to go in the league stage, Flick's Barcelona sit third, equal on points with fourth-placed Dortmund but ahead on goal difference. The top eight sides qualify automatically while teams ranked nine to 24 play an additional knockout round. 

The German coach faced Dortmund five times with Bayern, winning on each occasion. 

He told his Barcelona players to be ready for "an incredible atmosphere" at Dortmund's 82,000-strong Westfalenstadion, calling it "one of the most beautiful stadiums anywhere".

Dortmund coach Nuri Sahin earlier Tuesday praised his opposite number. 

"He is an exceptional coach who managed to bring his ideas to the team in a very short time."

"I don't think there's a favourite tomorrow. We're on the same level. We can win the game, but so can Barcelona."